I've read something on a1k.org (german forum) that someone with a 28mhz-card also experienced some lockups (system freezes). Jens responded to this that he had contact with an other guy with the same problem. This thing seems to happen with a rev 2b motherboard. Something to do with budgies? I really don't know what it means and don't understand much of it, but I do know Jens has a slightly modified version of the 28mhz card which he offers to that specific guy for testing purposes. Maybe you can check if you've got a rev 2b motherboard? I'm sure Jens can explain this better than I can and he can help out if necessary.
I did not have the time yet to test my 28mhz card.